1. Field of the Invention
The invention relates to performing positioning in a radio system.
2. Description of the Related Art
In CDMA (Code Division Multiple Access), a narrowband data signal of a user is modulated with a spreading code having a wider band than the data signal to a relatively wide band. Ln a WCDMA (Wideband CDMA) radio system, the bandwidth is yet considerably wider, because current mobile networks want to provide even more versatile services to users.
In radio systems, data transmission is done in duplex mode that can be implemented using the FDD (Frequency-Division Duplex) or TDD (Time-Division Duplex) principle. In an FDD solution, reception and transmission take place on different frequencies. In a TDD solution, reception and transmission take place at different time instants, but the frequency of the received signal and that of the transmitted signal are the same.
In WCDMA radio systems in particular, signal reception and transmission can be done with a diversity antenna that comprises an array of antenna elements. Signals are phased in relation to each other in a desired manner to make sure that the directional pattern formed by the antenna array is in the desired direction and shape. In practice, the phasing is done by multiplying the digital baseband antenna signal of each antenna element by complex coefficients that shape the directional pattern.
One of the most common positioning methods of a terminal is to measure the signal timing between the terminal and at least three base stations to find out the propagation delay of the signals between the terminal and each base station. The distance of the terminal from the base stations can be shown as a circle around each base station, because the direction of the terminal from the base stations is in most cases not known, and the radius of each circle represents the distance of the terminal from the base station. Each of the at least three circles has one common intersection point that is the location estimate of the terminal. Known measuring methods used in terminal positioning include the TOA (Time Of Arrival) measurement and the TDOA (Time Difference Of Arrival) measurement.
The TOA and TDOA positioning methods are, however, hampered by a problem caused by the distance of the subscriber terminal to the base station, i.e. near-far problem. In terminal positioning, this problem is also called a receiving problem. Other base stations do not hear a terminal that is close to one base station and the terminal does not hear the other base stations because of the interfering transmission of the nearby base station. When the signal propagation delay between the terminal and at least three base stations cannot be measured, neither can the position of the terminal be determined in this manner. In addition, methods based purely on delay measurements are often inaccurate, and it is difficult to find out the locations of the desired or first delay components in a multipath channel.
If a receiver comprises a diversity antenna or, more generally, at least a second antenna element, it is also possible to determine the location of a transmitter by means of the direction of reception. Usually, this is done by having several base stations measure the angle of arrival (AoA) of a signal transmitted by a terminal, whereby the location of the terminal can be determined to an intersection of two straight lines. Positioning is described in greater detail in publication: Overview of radiolocation in CDMA cellular systems, J. J. Caffery, Jr and G. L. Stuber, IEEE, 1998, which is incorporated herein as reference.
Determining the direction of reception is, however, a demanding task that requires good array antenna calibration and computationally demanding parameter estimation methods. Determining the direction of reception also requires a great deal of signalling from the transmitter to the base station and thus uses up a lot of capacity in the transmission direction from the terminal to the base station. In addition, the solution only enables positioning by a network element that requires the terminal to transmit something to the network element. There are, however, situations and services, in which the terminal must be capable of exact positioning, but the network element should not be able to position the terminal.